Generally, semiconductor devices are very small electronic components that are fabricated on a semiconductor wafer substrate. Using a variety of fabrication techniques, these devices are made and connected together to form integrated circuits. A number of integrated circuits may be found on one chip, and are capable of performing a set of useful functions in the operation of an electronic appliance. Examples of such electronic appliances are mobile telephones, personal computers, and personal gaming devices. As the size of these popular devices would imply, the components formed on a chip are extremely small.
There are many kinds of semiconductor components. Transistors, for example, are small switches that may be used to manipulate electrical signals. Diodes perform a similar though not identical function. Resistors and capacitors may also be formed as semiconductor devices. Over a million of such components may be formed on a single chip, and connected together to form the integrated circuits.
These semiconductor devices are fabricated on the wafer substrate using a sequence of operations. Generally speaking, ion implantation is used to impart semiconductor characteristics to the substrate or to structures formed on it. Layers of insulating and conducting material are then selectively added and removed to create the parts of each individual component. One common fabrication technique, for example, is called photolithography. In photolithography, a material called photoresist is formed on top of one or more layers of underlying material. The photoresist is then selectively exposed to light through a screen known as a mask. The exposed portions will possess different physical properties than the unexposed portions and, depending on the type of photoresist used, one of them may be removed by a selected solvent, leaving a set of protective structures.
Once the protective structures are in place, the unprotected portions of the underlying layers can be reduced or removed completely, for example by etching. They may also be subjected to treatments such as ion implantation. In any case, once the selective treatment or removal has been accomplished, the remaining photoresist structures may be removed without harming the underlying materials by a solvent selected for this purpose. An exemplary transistor will now be described as background to the present disclosure.
FIG. 1 is a side (elevation) view illustrating in cross-section a typical semiconductor device 10. Semiconductor device 10 is a transistor, which includes a gate structure 12 that has been formed on a substrate 20. Gate structure 12 includes a gate electrode 14 that is separated from the substrate 20 by a gate oxide 13. Gate electrode 14 may be made, for example, of a metal or a crystalline polysilicon (“poly”). The gate oxide 13 may simply be a portion of a silicon substrate 20 that has been encouraged to oxidize. A metal contact 15 may be used in the case of a poly gate to provide a reliable area for terminating external electrical conductors (not shown). A spacer may be positioned on either side of the gate electrode 14, and in this example spacer 16 and spacer 17 serve this function. A source region 21 and a drain region 23 are formed in the substrate 20, typically by ion implantation, and define a channel 22 between them and beneath gate structure 12. In operation, a current may be made to flow through channel 22 when a certain voltage is applied to the gate electrode 14.
At times, semiconductor devices such as the transistor of FIG. 1 may be used for different but complementary functions. One example is where two transistors are used, one as a core device and one as an I/O (input/output) device. FIG. 2 is a side view illustrating in cross-section a typical semiconductor device 30 of this kind. In semiconductor device 30, a core well 18 and an I/O well 19 have been formed adjacent each other, and are separated by STI (shallow trench isolation) structure 26 (also visible in FIG. 1). Core well 18 and I/O well 19 have been formed separately in substrate 20 by separate ion implantations in such a way as to give each its desired properties. The transistor of FIG. 1 is formed, in semiconductor device 30, on core well 18.
A second transistor, I/O device 40, has been formed on I/O well 19. I/O device 40 has a gate structure 45 that includes a gate electrode 34, which is separated from I/O well 19 by a gate dielectric 33. Contact 35 is disposed directly on gate electrode 34. Side spacers 36 and 37 are disposed on either side of gate structure 45. Source region 41 and drain region 43 define channel 42 in the I/O well 19 portion of substrate 20, under gate structure 45.
As should be apparent, the component parts of semiconductor device 10 and I/O device 40 are similar. There are differences, however, that affect the fabrication of such devices on the same wafer. The core device in some applications performs better when it has a thinner gate oxide than benefits the I/O device. The I/O device, on the other hand, may have to handle a higher voltage, however, so it cannot always be fabricated to the specifications best suited for the core device.
Needed, then, is a way to fabricate reliable semiconductor devices in an efficient manner despite their different performance criteria. The present invention provides just such a solution.